The present invention relates generally to a super-conductive coil in a synchronous rotating machine. More particularly, the present invention relates to an electromagnetic shield for a rotor having super-conducting field windings.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super-conductive rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K., to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have been cooled by liquid helium. After passing through the windings of the rotor, the hot, used helium is returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
Prior SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature, as a result no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
What is needed is a super-conducting field winding assemblage for an electrical machine that does not have the disadvantages of the air-core and liquid-cooled super-conducting field winding assemblages of, for example, known super-conductive rotors.
In addition, high temperature super-conducting (HTS) coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start up and shut down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor winding should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature and notwithstanding occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions.
SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. They are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system. These support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break.
Developing support systems for HTS coil has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy to fabricate components.
Structural supports for the HTS field coil windings has been one of the primary challenges to incorporating SC coils into rotors. The structure must support the SC coil winding without conducting substantial heat into the winding. The structure of the coil support has been minimized so as to reduce the mass that conducts heat from the rotor core into the cooled SC windings. However, minimizing the coil supports also limits the level of forces that can be withstood by the supports. If the forces that act on the rotor exceed the force carrying ability of the coil supports, then there is a substantial risk that the coil support will fail or that the coil windings will be damaged.
A potential source of forces that act on the rotor is torque due to grid faults. A High Temperature Super-Conducting (HTS) generator has a field winding SC coil is susceptible to electrical grid faults. A grid fault is a current spike in the power system grid to which is coupled the stator of the machine. Under grid fault conditions excessive current flows in the stator. This current causes an electrical disturbance in the stator winding that induces a strong transient magnetic flux into the rotor field winding coils.
The potential penetration of a transient magnetic field into the rotor field winding coil creates significant torque forces on the rotor coil winding and induces hysteresis and eddy current heating (alternating current loses) in the super-conducting field winding that my result in loss of super-conductivity. In addition, reducing these extraneous magnetic field penetrations will reduce alternating current (AC) loses in the super-conductor and preserve the super-conducting state of the rotor field winding. Minimizing the forces that act on a rotor allows for reduction in the structure of the coil support system. Reducing the rotor torque due to grid faults and other extraordinary variations in the electromagnetic field surrounding the rotor allows the coil support structures to be minimized.
Shielding the rotor prevents magnetic flux from the stator from interfering with the rotor. If a rotor field winding coil is not well shielded, the coil support must be reinforced to support the fault torque. An electromagnetic (EM) shield prevents stator magnetic flux from penetrating the rotor, which is more important for a super-conducting machine than a conventional machine.
The EM shield may cover nearly the entire surface of the rotor core. A cylindrical shield shape is useful for providing EM protection to the rotor. The EM shield may also served as the vacuum boundary for the SC coils. This boundary establishes a vacuum around the SC coil winding. The EM shield may be made of highly electrical conducted material, such as copper or aluminum.
The HTS rotor may be for a synchronous machine originally designed to include SC coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its SC coils are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines.
The coil support system is useful in integrating the coil support system with the coil and rotor. In addition, the coil support system facilitates easy pre-assembly of the coil support system, coil and rotor core prior to final rotor assembly. Pre-assembly reduces coil and rotor assembly time, improves coil support quality, and reduces coil assembly variations.
In a first embodiment, the invention is a rotor in a synchronous machine comprising: a rotor core; a super-conducting coil winding extending around at least a portion of the rotor core, said coil winding having a pair of side sections on opposite sides of said rotor core; and a conductive shield around the rotor core and covering said coil winding.
In another embodiment, the invention is a method for shielding a super-conducting coil winding on a rotor core of a synchronous machine comprising the steps of: assembling the coil winding and rotor core; attaching ends of the core to collars of end shafts coaxially aligned with the core, and installing a conductive shield around the rotor core, wherein the shield overlaps each of the collars.
Another embodiment of the invention is a rotor for a synchronous machine comprising: a rotor core having a conduit orthogonal to a longitudinal axis of the rotor; a race-track super-conducting (SC) coil winding in a planar race-track shape parallel to the longitudinal axis of the rotor; a tension rod inside the conduit of the core; a housing coupling the coil winding to the tension rod, and a electromagnetic shield around the rotor core.